The present invention relates to a switching power source, and particularly relates to a switching power source of the type in which an auxiliary power source is obtained from a main transformer.
In a switching power source of the type in which an auxiliary power source is obtained from a main transformer of the switching power source, a control circuit or the like for driving and controlling the ON-OFF operation of a switching element cannot be actuated when starting the switching power source because no auxiliary power source voltage is generated. Accordingly, the power source of this type requires means for making an auxiliary power source voltage rise when starting the switching power source.
For example, as shown in FIG. 1, a voltage divider circuit constituted by resistors 7 and 11 is connected to a DC input power source 1 so that an auxiliary power source smoothing capacitor 6 is charged by the voltage divider circuit so as to make an auxiliary power source voltage Vcc rise. The circuit of FIG. 1 is further provided with a transformer 2, a main switching element 3, and a control circuit 4, an auxiliary power source rectifier diode 5. The transformer 2 is provided with a primary winding 2-1, a secondary winding 2-2 for the auxiliary power source, and a secondary winding 2-3 for output. The output secondary winding 2-3 is connected to a rectifying/smoothing circuit (not shown) so as to supply DC output power to a load (not shown). An output voltage of the rectifying/smoothing circuit is fed back to the control circuit 4 and compared with a reference voltage provided in the control circuit 4. The control circuit 4 controls the switching operation of the main switching element 3 so as to make an error between the compared output and reference voltages zero. The DC input power source 1 may be obtained by rectifying and smoothing a commercial AC power source.
Further, as disclosed, for example, in JP-A-No. 61-211986, known is a switching power source provided with a starting circuit in which a voltage of a DC input power source obtained by rectifying/smoothing a commercial AC power source is divided by capacitors (practically, a resistor is connected in parallel to each of the capacitors) so as to supply a voltage to a circuit for driving the ON-OFF operation of switching elements of an invertor circuit (that is, a circuit for amplifying an output of a control circuit to thereby directly drive the ON-OFF of switching elements of an invertor circuit). The JP-A-No. 61-211986 describes that by using such a starting circuit the switching power source can be made smaller in size and lower in cost than the conventional switching power source in which an output of a transformer for a control circuit connected directly to a commercial AC power source is rectified so as to be used not only for the control circuit but for starting the above-mentioned circuit for driving the ON-OFF of the switching element.
In the latter prior art, however, there is a problem in that although the control circuit can be started by a voltage obtained by the divider circuit to thereby make an auxiliary power source rise, the voltage is kept applied to the divider circuit even after the rise of the auxiliary power source, resulting in power loss.
In the case of the circuit shown in FIG. 1, there is a problem. Since the auxiliary power source is arranged so as to be supplied by the transformer winding 2-2 after starting, it is necessary to make the quantity of charge by the resistor 7 small in advance. To this end, the value of the resistor 7 is selected to be large. On the other hand, the smoothing capacitor 6 is selected to have a large capacitance in order to make the auxiliary power source voltage Vcc stable. Accordingly, the charging time of the smoothing capacitor 6 in starting is so long that it takes a long time before the switching operation is started. Further, in the case where the switching power source as shown in FIG. 1 is used as a power source for a load such as a magnetron, in which a load voltage varies in accordance with the operational conditions, there is a problem in that the auxiliary supply voltage varies in proportion to the variation of the load voltage.